"The North's rejection was made during a border contact between liaison officials of Red Cross authorities from both sides. North Korea did not elaborate on the reason."

The border between the two Koreas is the world's most heavily fortified frontier, dividing about 600,000 South Korean troops from North Korea's 1.1 million-strong army.

Security risk?

"We do not know the exact reason, but we just presume that North Korea might be concerned about security issues involved in allowing cross-border transportation," a Unification Ministry official said.

Red Cross officials said delivering aid by road to the blast site at Ryongchong would take about four hours. The alternative, sea transportation, would take nearly 48 hours.

The Red Cross spokesman said the refusal by North Korea might not be the final word.

North Korea has asked for a meeting of Red Cross officials from the two sides to discuss "technical details" of the proposed transportation at the town of Kaesong, just over the border inside North Korea, on Tuesday.